As the electronic industry advances to a high level, integration density of semiconductor devices increases. Higher integration of semiconductor devices is an important factor in determining product price. In other words, as integration density of semiconductor devices increases, product prices of semiconductor devices may decrease. Accordingly, requirement for higher integration of semiconductor devices is increasing. Typically, since integration of semiconductor devices is mainly determined by the area occupied by a unit memory cell, integration is greatly influenced by the level of fine pattern forming technology. However, pattern fineness is limited due to extremely expensive semiconductor equipments and/or difficulties in semiconductor fabrication processes.
To overcome such a limitation, there have been recently proposed three-dimensional semiconductor memory devices. However, since the foregoing new structure causes limitations such as process instability and/or lowering in product reliability, many studies are being conducted to overcome these limitations.